Ultrasonic Rheology of Mixed-Phase Systems Using a QCM as an Effective Rheological Device – Theory and Applications for Combined Surface and Bulk Rheology

Rheology provides a powerful way to access molecular scale data from a macroscopic measurement. Ultrasonic rheology, performed with a quartz-crystal microbalance (QCM) instrument, provides a straightforward route to sensitive (< 0.1 mPa·s STD) rheology of either bulk or surface phases. Presented is the necessary theory (Johannsmann and Voinova models) to perform such measurements, and some applications of that theory - including rheology on polymer brush layers, lipid vesicle adsorption, proteins in the bulk phase, and microemulsions. In each of these applications, an advantage of QCM-based rheology is described, such as small sample size, precision and the ability to perform either bulk or surface measurements. Additionally, corrections to the standard Sauerbrey approximation, which often results in an underestimation of adsorbed mass, are presented. In conclusion, the QCM is shown to provide a facile route to rheological measurements in the MHz regime.